Elastomeric polymers, particularly polyurethane elastomers, are becoming increasingly important in commercial applications. Unlike many other thermoplastic and thermoset elastomers, a wide variety of raw materials may be employed in the preparation of polyurethane elastomers, thereby allowing the preparation of elastomers with physical properties geared to the intended application. Thus, there are a myriad of diverse applications for polyurethane elastomers including, for example, caulks and sealants, elastomeric fibers, viscoelastic and energy absorbing materials, and extrusion and injection moldable articles such as gears, automobile fascias, and ski boots.
Spandex is a particular elastomeric polymer in fiber form which is a well-known component of clothing, particularly sportswear, which adds stretch to the clothing. Spandex is a urethane-containing polymer composed of alternating soft and hard regions within the polymer structure. Generally speaking, there are three methods for manufacturing fibers, including Spandex fibers, from polyurethane polymers: (1) dry-spinning; (2) wet-spinning, and (3) melt-spinning. Both dry-spinning and wet-spinning involve the use of organic solvents which, due to health, safety and environmental considerations, is preferably avoided. Melt-spinning is thus advantageous inasmuch as it does not involve the use of solvents.
Prior art polyurethanes are often prepared by the "one-shot" method. This method involves reacting together, at the same or about the same time, the various reaction components including, for example, polyol, diisocyanate compound and chain extender. The one-shot method often leads to polyurethanes, particularly polyurethanes in which the chain extender includes aromatic moieties, which typically contain higher melting material, i.e., material having melting points of, for example, about 225.degree. C. or greater, including above about 235.degree. C. The use of melt-spinning temperatures of greater than about 225.degree. C. may therefore be required during melt-spinning of prior art polyurethanes. However, the polyurethanes themselves are generally thermally unstable at temperatures above about 225.degree. C. Thus, the use of higher temperatures during melt-spinning may result in thermal decomposition of the polyurethane, whereas the use of lower melt-spinning temperatures may result in plugging or clogging by the higher melting material of the melt-spinning apparatus. Accordingly, polyurethanes generally cannot be melt-spun with the continuity required for commercial operations if they contain any higher melting material. As a consequence, polyurethanes generally must be dissolved in a solvent and either dry or wet spun into fibers which, as noted above, poses safety and environmental concerns.
In attempting to address the aforementioned problems with prior art polyurethanes, polyurethanes have also been prepared in which the involved chain extender is a compound which contains no aromatic moieties, i.e., an aliphatic compound. However, these polyurethanes are generally characterized by undesirable chemical and/or physical properties, including tackiness, low melting temperatures and low heat distortion temperatures. Products melt-spun from these polyurethanes are readily prone to melting or to becoming misshapen upon exposure to slightly elevated temperatures. To improve their heat resistance, prior art melt-spun polyurethanes have been crosslinked during and/or after the spinning process. Crosslinking in melt-spinning processes is generally undesirable in that it adds complexity and cost to the spinning methods. Unexamined Japanese application No. SHO 63(1988)-317238 describes an alternate crosslinking process in which unreacted ingredients are injected into the spin cell to complete the crosslinking reaction. This process is also undesirable in that it leaves the vam only partially cured as it is wound during spinning and it must be permitted to cure before rewinding and/or shipping.
Accordingly, new and/or better alternatives to prior art methods and elastomeric polymers for use in the manufacture of melt-spun fibers are needed. The present invention is directed to these as well as other important ends.